Strangles is diagnosed either directly by detection of S. equi itself or indirectly by detection of rising levels of antibody against S. equi in blood samples, although presence of antibodies against S. equi does not necessarily indicate that an animal is still infectious to other horses.
Direct detection of S. equi is either by laboratory isolation or by qPCR detection of its DNA from nasopharyngeal swabs, abscess contents and/or guttural pouch washes. It should be noted that low bacterial numbers, the concurrent presence of the closely related S. zooepidemicus or recent antibiotic treatment, may make the detection of S. equi by culture more difficult and less sensitive than qPCR.
When taking nasopharyngeal swabs, it is particularly important to sample the back of the pharynx around the opening of the gutteral pouch, using specially designed elongated swabs with enlarged absorbent heads1. There is no need to use smaller, guarded swabs as the main purpose of swabbing for strangles is to optimise the chances of detecting the organism if it is present. Shedding of S. equi into the nasopharynx often occurs intermittently, so repeated swabbing is recommended to confirm negative results. S. equi should be more reliably confirmed or excluded following testing by qPCR or culture and qPCR of frank pus from obvious draining abscesses.
The carrier state may be diagnosed or excluded by sequential nasopharyngeal swabs or, preferably, endoscopic examination (‘Scoping’) of the guttural pouches and submission of guttural pouch washes for testing by qPCR alone or by culture and qPCR. A series of three nasopharyngeal swabs, collected one week apart, will result in detection, by positive qPCR, on at least one of the swabs in >90% of carrier horses. As the sensitivity of S. equi detection for identifying guttural pouch carriers on three nasopharyngeal swabs is broadly equivalent to testing bilateral guttural pouch samples and a single nasopharyngeal swab taken on the same one occasion, the latter approach is the recommended sampling protocol for determining infectious status in seropositive, asymptomatic horses.
Although carriers only shed S. equi intermittently, over 90% of carriers maintain specific antibodies in their blood. These antibodies can be detected by a blood ELISA test, which may provide a useful tool to help identify carrier animals. Newly exposed horses take at least two weeks to develop sufficient antibodies to give a positive blood ELISA result and may remain positive for up to six months after recovery. As with all ELISA tests, false negative (7% based on 93% sensitivity) and false positive (<1% based on >99%specificity) results may occur (Robinson et al., 2013). Therefore, results must be interpreted carefully and in the context of the specific situation in which they are being used. Your veterinary surgeon may obtain a more detailed description of the use and interpretation of the strangles blood test from the testing laboratory.